1. Field of the Invention
The present invention relates generally to a circuit for removing jitter of a chrominance signal, and more particularly, to a circuit for removing a jitter component from a chrominance signal included in a color video signal.
2. Description of the Background Art
As shown in FIG. 1, a video system is conventionally well-known in which video signals output from a video signal source 1 such as a video tape recorder or a laser disk player are applied to a television set 2 wherein the video signals are displayed on a CRT screen. In such a video system, fluctuation in time is caused in the video signals reproduced by the video signal source 1 due to various causes such as irregular rotation of a disk motor and vibration of a magnetic tape in travelling. Such fluctuation in time of a video signal is referred to as "jitter", which has various adverse effects on the picture displayed in the television set 2. Particularly, the jitter components included in chrominance signals results in uneven color of the display picture to make the display frame extremely unclear.
Therefore, various measures have been conventionally adapted to remove jitter components from chrominance signals in the video signal source 1.
FIG. 2 is a block diagram showing a schematic arrangement of a reproduced signal system of a conventional video tape recorder. Generally, a video tape recorder is provided with a frequency converting circuit for converting a low frequency chrominance signal reproduced from a magnetic tape to a chrominance subcarrier frequency adaptable to a television set. The video tape recorder shown in FIG. 2 is provided with a jitter removing circuit for removing jitter component from chrominance signals by using a frequency conversion function of this frequency converting circuit.
In FIG. 2, a video signal reproduced from a magnetic tape 3 by a magnetic head 4 is amplified by an amplifier 5 and then applied to an FM luminance signal separating circuit 6 and a low frequency chrominance signal separating circuit 7. The FM luminance signal separating circuit 6 separates frequency-modulated luminance signal components included in the reproduced video signal and applies the same to an FM demodulator 8. The FM demodulator 8 demodulates the frequency-modulated luminance signal and applies the demodulated luminance signal to one input of a mixer 9. Meanwhile, the low frequency chrominance signal separating circuit 7 separates a low frequency chrominance signal from the reproduced video signal and applies the same to one input end of a multiplier 10. An output signal of a voltage controlling oscillator (hereinafter referred to as VCO) 11 is applied to the other input end of the multiplier 10. The multiplier 10 multiplies the low frequency chrominance signal separated by the low frequency chrominance signal separating circuit 7 and the output signal of the VCO 11 together and applies the multiplied signal to a bandpass filter 12. A pass band of the bandpass filter 12 is selected to have a center frequency set to a chrominance subcarrier frequency fsc. The chrominance signal band-width-limited by the bandpass filter 12 is applied to the other input end of the mixer 9 and one input end of a phase comparator 13. An output signal of a crystal oscillator 14 is applied to the other input end of the phase comparator 13. An oscillating frequency of the crystal oscillator 14 is set to a chrominance subcarrier frequency fsc. The output of the FM modulator 8 is applied also to a synchronization separating circuit 15. The synchronization separating circuit 15 separates a horizontal synchronizing signal from the output of the FM demodulator 8, that is, the demodulated luminance signal and applies the same to a burst gate pulse generating circuit 16. The burst gate pulse generating circuit 16 generates a burst gate pulse defining a period of a color burst signal in a chrominance signal by delaying the horizontal synchronizing signal applied from the synchronization separating circuit 15 and applies the same to the phase comparator 13. The phase comparator 13 compares phases of the output signal of the bandpass filter 12 and the output signal of the crystal oscillator 14 only in a period the color burst signal is included, using the burst gate pulse applied from the burst gate pulse generating circuit 16 as a timing signal. The output of the phase comparator 13 is applied to the VCO 11 through a low-pass filter 17. An oscillating frequency of the VCO 11 fluctuates, centered at fsc+629 Khz (629 Khz is a frequency of a low frequency chrominance signal), in response to the output voltage of the low-pass filter 17.
In the video tape recorder having the above-described arrangement shown in FIG. 2, the multiplier 10 multiplies the low frequency chrominance signal of 629 KHz and the output signal of the VCO 11 having a frequency of fsc+629 KHz, so that chrominance signal component having fsc is included in the output signal of the multiplier 10. The chrominance signal component of fsc is extracted by the bandpass filter 12. Accordingly, the frequency of the chrominance signal separated from the video signal is converted from 629 KHz to fsc. The chrominance signal output from the bandpass filter 12 is mixed with the luminance signal demodulated by the FM demodulator 8 in the mixer 9 to be output from an output terminal 18. Meanwhile the phase comparator 13 compares phases of the color burst signal included in the chrominance signal output from the bandpass filter 12 and a reference frequency signal of fsc output from the crystal oscillator 14 and generates a control voltage corresponding to the phase difference. The control voltage is applied through the low-pass filter 17 to the VCO 11, wherein a voltage value of the control voltage fluctuates in response to the jitter component included in the chrominance signal. Accordingly, the oscillating frequency of the VCO 11 also fluctuates in response to the jitter component included in the chrominance signal. Therefore, the jitter components in the chrominance signal is canceled in the multiplier 10, so that the jitter component is removed from the chrominance signal.
FIG. 3 is a block diagram showing a schematic arrangement of a conventional television set including a so-called color synchronization circuit. The color synchronization circuit is a circuit for, in demodulating a chrominance signal in a television set, synchronizing a phase of a chrominance subcarrier signal which is to be a reference signal with a phase of the chrominance signal to be demodulated. The arrangement of the conventional television set shown in FIG. 3 will be described in the following.
In FIG. 3, the high frequency signal induced in an antenna 21 is applied to a tuner 23 through an antenna terminal 22. The tuner 23 receives a television video signal of a desired broadcasting station and outputs the same. The output signal of the tuner 23 is converted and amplified to have an intermediate frequency by an intermediate frequency amplifying circuit 24 and the applied to a terminal A of a switch SW1. Meanwhile, a video terminal 25 receives the video signal output from the video signal source 1 of FIG. 1. The video terminal 25 is connected to a terminal B of the switch SW1. The output signal of the switch SW1 is amplified by an amplifier 26 and then applied to a luminance signal separating circuit 27 and also to a chrominance separating circuit 28a and a synchronization separating circuit 28b in a color synchronization circuit 28. The luminance signal separating circuit 27 separates a luminance signal Y from the video signal applied from the amplifier 26 and applies the same to a matrix circuit 29.
The color synchronization circuit 28 includes the chrominance separating circuit 28a, the synchronization separating circuit 28b, a burst gate pulse generating circuit 28c, a phase comparator 28d, a low-pass filter 28e, a VCO 28f and a crystal oscillator 28g. The chrominance separating circuit 28a separates a chrominance signal from the video signal applied from the amplifier 26. The output of the chrominance separating circuit 28a is applied to a color demodulating circuit 30 and also to one input end of the phase comparator 28d. The synchronization separating from the video signal applied from the amplifier 26. The horizontal synchronizing signal output from the synchronization separating circuit 28b is applied to the burst gate pulse generating circuit 28c. The burst gate pulse generating circuit 28c delays the applied horizontal synchronizing signal to generate a burst gate pulse defining a period when a color burst signal is included in the chrominance signal. The burst gate pulse output from the burst gate pulse generating circuit 28c is applied to the phase comparator 28d as a timing signal for controlling timing for comparing of phases. The output of the phase comparator 28d is applied to the VCO 28f through the low-pass filter 28e. The VCO 28f has an oscillating frequency fluctuating in response to the control voltage from the low-pass filter 28e, with the oscillating frequency fsc of the crystal oscillator 28g as a center frequency. The output signal of the VCO 28f is applied to the color demodulating circuit 30 and also to the other input end of the phase comparator 28d. The color demodulating circuit 30 demodulates the chrominance signal applied from the chrominance separating circuit 28a, with a chrominance subcarrier signal applied from the VCO 28f as a reference signal to output color difference signals R-Y and B-Y. These color difference signals R-Y and B-Y are applied to the matrix circuit 29. The matrix circuit 29 generates three primary color signals R, G and B based on the luminance signal Y applied from the luminance signal separating circuit 27 and the color difference signals R-Y and B-Y applied from the color demodulating circuit 30 and applies the same to a CRT 31. The CRT 31 displays the contents corresponding to the applied three primary color signals R, G and B.
Operations of the conventional television set shown in FIG. 3 will be described. When the switch SW1 is turned to the terminal A, the video signal received by the tuner 23 is applied to the amplifier 26 through the intermediate frequency amplifying circuit 24. Conversely, when the switch SW1 is turned to the terminal B, the video signal input from the external video signal source 1 (see FIG. 1) is applied to the amplifier 26. The video signal output from the amplifier 26 is applied to the luminance signal separating circuit 27 and the chrominance separating circuit 28a to be separated into a luminance signal Y and a chrominance signal. The separated luminance signal Y is applied to the matrix circuit 29 without conversion. Meanwhile, the separated chrominance signal is applied to the color demodulating circuit 30 wherein the chrominance signal is demodulated and converted into the color difference signals R-Y and B-Y, the color demodulating circuit 30 carrying out demodulating operation of the chrominance signal with a chrominance subcarrier signal as a reference signal. At this time, the chrominance subcarrier signal used as a reference signal for the demodulating operation is required to have a phase synchronizing with the chrominance signal to be demodulated, that is, the burst signal of the chrominance signal separated by the chrominance separating circuit 28a. Therefore, the color synchronization circuit 28 is provided in order to synchronize the phase of the chrominance subcarrier signal as a reference signal with the phase of the burst signal to be demodulated. In the color synchronization circuit 28, the phase comparator 28d detects a phase difference between the burst signal from the chrominance separating circuit 28a and the output signal of the VCO 28f (chrominance subcarrier signal), so that the oscillating frequency of the VCO 28f is controlled in response to the phase difference. As a result, the phase of the chrominance subcarrier signal applied as a reference signal from the VCO 28f to the color demodulating circuit 30 always synchronizes with that of the burst signal output from the chrominance separating circuit 28a. The phase comparator 28d performs the phase comparing operation only when the burst gate pulse applied from the burst gate pulse generating circuit 28c is output, that is, when the color burst signal is included in the chrominance signal. The luminance signal Y separated by the luminance signal separating circuit 27 and the color difference signals R-Y and B-Y output from the color demodulating circuit 30 are converted into the three primary color signals R, G and B in the matrix circuit 29 and applied to the CRT 31. Accordingly, the CRT 31 displays the contents corresponding to these three primary color signals R, G and B.
The color synchronization circuit 28 shown in FIG. 3 is disclosed in, for example, Japanese Patent Laying-Open No. 62-183292.
As shown in FIG. 2, a conventional video tape recorder is provided with a jitter removing circuit to remove jitter component of a chrominance signal. However, it is difficult to remove the jitter components completely from the chrominance signal only by such a jitter removing circuit as shown in FIG. 2. Accordingly, some of the jitter component is not removed, which is applied to the television set 2.
The color synchronization circuit 28 in the conventional television set shown in FIG. 3 has a time constant of a loop set to a long period of about 30 H (H is one horizontal scanning period) so as to perform a comparatively stable operation even in a weak electric field, which is directed to the prevention of disturbance in color synchronization even when the color burst signal is temporarily buried in the noise component in a weak electric field. Accordingly, the color synchronization circuit 28 performs an average phase control for a period of 30 H but can not follow the color jitter component of a relatively short period shorter than 30H. Therefore, the jitter component which could not be removed in the jitter removing circuit in FIG. 2 results in disturbance in hue on a picture frame of the CRT, causing deterioration of the picture quality.
Such a color jitter component of a relatively short period as described above is often included in a reproduced output color video signal of a laser disk player or a video tape recorder. Therefore, the picture quality in the reproduction is deteriorated particularly in the case where a laser disk player or a video tape recorder is used as a video signal source 1. In order to resolve the above-described problems, proposed is a provision of an additional jitter removing circuit in some part of the video system shown in FIG. 1. This additional jitter removing circuit removes the jitter component which could not be removed by the jitter removing circuit provided in such a video tape recorder as shown in FIG. 2.
FIG. 4A is a theoretical block diagram showing a common arrangement of a jitter removing circuit additionally provided as described above. In the drawing, a chrominance signal having a frequency of fsc+.DELTA.f is applied one input end of a multiplier 41, wherein fsc denotes a chrominance subcarrier frequency and .DELTA.f denotes jitter component. Jitter components of extracted from the original chrominance signal by some way is applied to the other input end of the multiplier 41. Thus, the multiplier 41 multiplies the chrominance signal including the jitter component .DELTA.f and the jitter component .DELTA.f together. At this time, the output signal of the multiplier 41 includes frequency component of (fsc+.DELTA.f).+-..DELTA.f and frequency component of fsc+.DELTA.f, wherein the frequency component of fsc+.DELTA.f is a leak signal in the multiplier 41. A bandpass filter 42 has a pass band having a center frequency of a chrominance subcarrier frequency fsc and limits the band width of the output signal of the multiplier 41.
FIG. 4B is a diagram showing a relationship between the output of the multiplier 41 and the pass band of the bandpass filter 42 in FIG. 4A. As shown in the diagram, the output signal of the multiplier 41 mainly includes three frequency components, that is, fsc, fsc+.DELTA.f and fsc+2.DELTA.f. As shown in FIG. 4B, these three frequency components are very close to each other. Meanwhile, the pass band characteristic of the bandpass filter 42 cannot be made extremely sharp because the pass band should be wide to some extent in order to cover frequency deviation of the chrominance signal. Therefore, the three frequency components included in the multiplier 41 are included in the pass band of the bandpass filter 42, so that it is not possible to extract the frequency component of fsc only. Accordingly, the jitter component is not removed but output from the bandpass filter 42.